The present invention relates generally to data communications networks and more specifically to an enhancement to network interface units in a high speed data interface network.
Modern data networks allow for the high speed transmission of digital data between multiple users. Such data might be digitized voice data in a private branch exchange (PBX) office telephone network, video data, or information data from digital information processing systems such as personal computers. Modern networks may allow for the transmissions of all three types of data over the same physical network cable.
One configuration for such a network is described in PCTUS89/01806 application. This network is illustrated in FIG. 1. It is characterized by a single network medium 12 connected to a plurality of network nodes, represented by Network Interface Units (NIUs) 20 each connected to a phone, computer, or other video, data, or voice processing device (P) 22. The single medium is such that it can simultaneously carry a number of different signals, each in a different frequency band, thus allowing the network to have a number of frequency division multiplexed channels for carrying data. Each channel is further divided into two different frequency bands: a transmit frequency band and a receive frequency band. NIUs on the network transmit data on the transmit frequency and receive it on the receive frequency. A single head-end retransmission unit (HRU) 50 receives all data transmitted on the transmit frequency and retransmits it on the receive frequency for reception by all network nodes.
FIG. 2 is a diagram showing the time structure of signals on this network. Circuitry associated with the HRU provides a series of timing mark packets (TMs), transmitted simultaneously on all channels, at 1-ms intervals, thereby defining a series of 1-ms frames.
In one configuration of the network for which the present invention is designed, each frame consists of a 10-byte TM, a 71-byte (60 data bytes) signalling packet (SP), and 28 19.5-byte (16 data bytes) voice/data timeslots (VTSs), each capable of containing a voice/data packet (VP).
VPs are used to provide voice communication containing binary encoded (pulse code modulation--PCM) speech from a specific phone conversation or to provide data communication. They are transmitted every cycle during the course of a conversation. When transmitting voice, VPs contain no computer recognizable information. They are merely reconstructed into voice at the receiving node.
SPs are used for communications between nodes and contain computer recognizable information pertinent to the control of the network. Among the functions of the SPs is the determination of skew intervals.
Each node is characterized by a skew time related to its physical position on the bus. Skew time refers to the different propagation delays resulting from the fact that the different nodes are at different distances from HRU 50. The nodes most remote from the HRU will receive the timing marks latest in time, and would, if they merely synchronized their transmissions to the timing mark, transmit relatively late compared to nodes nearer the HRU.
As a result of this skew time, there is a limit to the geographic range of the network. A typical maximum range in the prior art is about 5 km.